1. Field of the Invention
The present invention relates to a gravure coating apparatus, and in particular, to a gravure coating apparatus which forms stripe-shaped coating film by means of a gravure roll having a groove between plate cylinders.
2. Description of the Related Art
In recent years, batteries have been increasingly used as a power source of various devices. Accordingly, there has been a strong demand for an increase in the capacity and output power of batteries and for a reduction in the thickness and weight thereof. Lithium ion rechargeable batteries, lithium polymer rechargeable batteries, nickel metal hydride batteries, and the like have been used as batteries capable of satisfying the demand. These batteries are being improved to further increase the capacity and output power and also to reduce the cost and stabilize the life.
As an electrode plate for such batteries, there is known an electrode plate constituted by forming coated portions containing an active material on one or both sides of a metal core serving as a collector. The coated portion is formed by coating the surface of the core with a paste containing the active material. In order to reduce manufacturing cost in a method for manufacturing such an electrode plate, various methods have been known (see, for example, Japanese Patent Laid-open Publication No. 2001-327906). Specifically, while a strip-like core is fed in its longitudinal direction, a plurality of stripes of a coating layer are formed so as to be separated in the width direction of the core with an uncoated portion serving as a lead portion formed between the stripes. After the coating layer is dried and compressed, the core is subjected to cutting, whereby an electrode plate having a desired size is manufactured with high productivity.
Furthermore, in various rechargeable batteries, a separator formed of a porous thin film sheet is disposed between positive and negative electrode plates in order to insulate the electrode plates from each other and to hold an electrolytic solution. In order to prevent foreign materials from piercing the separator or to prevent an internal short circuit from occurring and spreading when heat damage occurs, a method has been proposed in, for example, the pamphlet of International Publication No. WO2005/081336, In this method, a porous insulating film, which serves as a protection film for the coated portion and which contains an inorganic oxide filler such as alumina powder and a resin binder, is formed on the surface of a coated portion of an electrode plate. Specifically, the protection film is coated by use of a gravure roll and is formed such that a lead formation portion is not coated.
Furthermore, as a method for forming an active material layer on the surface of a collector, a method is known in which an active material layer is formed by printing a coating solution in a pattern by means of a gravure printing method (see, for example, Japanese Patent Laid-Open Publication No. Hei 9-298058).
Hence, it has been conceived that a technique similar to those described in the abovementioned pamphlet of is International Publication No. WO2005/081336 and Japanese Patent Laid-Open Publication No. Hei 9-298058 is utilized in an apparatus for forming a protection film composed of a porous insulating film on the surface of the abovementioned stripe-shaped coated portions formed on the surface of a strip-like core. That is, a protection film is formed by means of gravure coating. Specifically, as shown in FIGS. 3 and 4, a strip-like electrode plate 11 having a plurality of coated portions 13 formed in stripes on the surface of a core 12 is guided by guide rollers 14 and is fed in its longitudinal direction. A coating solution 17 containing an inorganic oxide filler and a binder is applied to plate cylinders 16 of a gravure roll 15. Then, the coating solution 17 in the plate cylinders 16 is transferred to the surface of each of the coated portions 13 to thereby form a protection film 18.
However, it has been found that, when the coating solution 17 is transferred from each of the plurality of the plate cylinders 16 provided in the gravure roll 15 to the plurality of the stripe-shaped coated portions 13, the coated amount of the coating solution 17 is large at both the side edge portions of each of the coated portions 13, as shown in FIG. 5. Since the protection film 18 cannot be formed uniformly over the entire surface of each of the coated portions 13, the length of a transportation path of ions becomes large in the thick portions of the protection film 18, and the power generation function of the electrode plate 11 deteriorates, thereby causing a problem that the output power, capacity, and life of a battery is adversely affected.
As above, the cause of the increase of the coated amount of the coating solution 17 in the side edge portions of the coated portions 13 has been investigated. Consequently, it has been found that the increase is caused since the coating solution 17 tends to be collected in the edge portions of the plate cylinders 16. Hence, as shown in FIGS. 6A and 6B, a scraper 19 is provided to scrape the coating solution 17 sticking to an edge surface 16a of the plate cylinder 16 of the gravure roll 15. However, the scraper 19 made of a polyethylene terephthalate (PET) thin plate must be appropriately pressed against the edge surface 16a of the plate cylinders 16. Therefore, as the scraper 19 wears, the pressing pressure must be controlled according to the wear. However, it is difficult to place a pressing pressure mechanism in a narrow groove between the plate cylinders 16, and the scraper 19 must be replaced frequently since it wears rapidly. Therefore, a problem arises in that the productivity deteriorates due to the replacement operations.